Game director Aaron Keller spoke about this while talking to journalists. According to him, previous story missions and the “Archives” events generated high interest, although they failed to retain the audience.
When you look at the campaign we released, you realize that despite the big hype and story progression, people didn’t play it the way we would have liked.
Creating PvE missions requires significantly more time and resources than other event formats. That is why Blizzard switched to animated shorts and comics, directing the freed-up resources toward developing the competitive aspect of Overwatch.
At the same time, Keller did not rule out the return of PvE: “I would never rule that out.” However, in his view, a full-scale campaign would need to become a “completely new game” with a dedicated team.